image: truecolor to palette

Hello

I am trying to reduce a truecolor image to a palette one.
imagetruecolortopalette() works just fine, except, that I would like to
keep a few color unchanged. like a (255,0,0) red, a (255,255,255) white
.... and so on.

My truecolors images are somewhat greenish, with only a tiny bit of red
or white, so when I use imagetruecolortopalette(), those colors are
lost.

It would be great to initialise the destination palette with a few
colors, and then let imagetruecolortopalette() do the job for the rest
of the palette.

Is it possible ?

Thanks

.